1. Technical Field
This invention relates to a process for producing lithium from recycled lithium-containing alloys such as aluminum-lithium alloy scrap.
2. Background of the Prior Art
Lithium is the lightest among the elements that are solid under normal conditions, having a specific weight of about 0.59 g/cm.sup.3. This light metal floats on water and oil and also on many molten salts. Lithium was produced for the first time by the electrolysis of lithium chloride. Other starting materials such as lithium bromide or lithium hydroxide have been found to be less desirable in the production of lithium metal for reasons that they are more expensive and that secondary reactions produce low current efficiencies. Lower operating temperatures in the production of lithium from lithium chloride can be achieved by adding other compounds to form low melting point eutectics. For example, a low melting point eutectic is achieved by formulating a composition of LiCl/KCl of about 57/43 mol %. Even lower melting point eutectics are available from compositions such as LiCl/NaH.sub.4 Cl and several LiCl/LiNO.sub.3 compositions.
In the production of lithium electrolytically from lithium chloride, potassium chloride has been added to the electrolyte melt because of a decomposition potential higher than that for lithium chloride. Decomposition potentials at about 450.degree. C. are 3.68V for LiCl, 3.57V for NaCl, and 3.81V for KCl. Decomposition potentials indicate that potassium will not be reduced as readily as lithium or sodium. Sodium, on the other hand, has a decomposition potential which indicates that sodium is reduced preferentially to lithium. For this reason, sodium contaminants should be eliminated before attempting the production of pure lithium from lithium chloride.
As early as the earliest patents for the production of aluminum electrolytically, lithium compounds were mentioned for use in aluminum electrolysis, but financial aspects limited the universal realization of this proposal until only recently. Lithium fluoride can be used, but lithium carbonate is more economical and reacts under the prevailing conditions with aluminum fluoride to form lithium fluoride and aluminum oxide. The main advantage of lithium addition in aluminum electrolysis is attributable to a reduction in the liquidus temperature of the electrolyte and reductions of melt density and viscosity. A drawback, however, is a reduced solubility for the aluminum oxide. Nevertheless, this drawback can be compensated by reduced calcium fluoride content. Lower temperatures in the electrolysis eventually would freeze the bath, but this can be counteracted by increasing the cell current or by increasing the anode-cathode distance. Lithium compounds have been added to aluminum electrolysis in the amounts of 3 to 5% as lithium fluoride to reduce costs by lower energy consumption by about 3%, to reduce carbon consumption by about 2%, to reduce bath addition by up to 4%, and to reduce fluoride emissions by up to about 50%.
Lithium metal can be produced directly from ore by heating together a mixture of spodumene, lime, and aluminum or silicon to a temperature above about 100.degree. C. at a pressure of 25 microns to produce a lithium-magnesium alloy containing 85-90% lithium. The technical literature has mentioned a method for producing lithium metal by an electrolysis of a fused mixture of lithium and potassium chloride. The technical literature also mentions that electrolysis is employed to form alloys of lithium with metals of low melting points, such as lead, tin, or zinc, wherein the molten heavy metal is used as the cathode and the lithium is electrodeposited from a fused salt mixture containing lithium chloride.
The electrolytic production of lithium alloys has been investigated for alloys of lead, zinc, aluminum, magnesium, and copper-aluminum from starting materials consisting of a fused mixture of equal weights of lithium chloride and potassium chloride. Cathodes of the various alloying metals in the molten state were used.
Pure lithium has been obtained from lead-lithium and copper-aluminum-lithium alloys by distillation at low pressures. Lithium metal has been produced from lithium chloride using electrolytes of lithium chloride and potassium chloride, and, optionally, lithium bromide at a current efficiency of over 90% and a metal yield on the basis of lithium chloride input of 95% in a sodium type of cell. The sodium type of cell has anodes of graphite and cathodes of steel. The cell is heated externally by gas or oil, and the salt mixture is fused between anodes of graphite and cathodes of steel. Current is applied and molten lithium metal is formed. Chlorine is formed at the anode and is vented and recovered from the cell, five pounds of chlorine for each pound of lithium metal produced.
U.S. Pat. No. 3,962,064 discloses lithium formed on a cathode and collected in an electrolytic tank of stainless steel having a solid cathode and anode. An inert gas is introduced into a discharge compartment, and lithium is transferred to that chamber for casting into ingots under inert atmosphere.
Japanese Patent Disclosure No. 79,043,811 discloses a production method for metallic lithium in which mixed salts consisting of 42-52 wt % lithium chloride and 58-48 wt % potassium chloride are electrolyzed in the molten state. The electrolysis is carried out in the molten state by heating the mixed salts at 380.degree.-500.degree. C. at a voltage of 4-12V and a current density of 50-300 A/dm.sup.2. A lithium electroconductive solid electrolyte such as a lithium sulfate, a lithium-beta-alumina, or a lithium alumina silicate are used as a diaphragm for partitioning the cathode and anode chambers used in the electrolysis. When the content of lithium chloride is less than 42 wt %, the ratio of metallic lithium redissolved is markedly increased and electrical resistance is also increased. When the content of lithium chloride exceeds 52 wt %, these same effects occur. Electrodes used in the electrolysis are graphite or carbon for the anode and stainless steel for the cathode.
U.S. Pat. No. 4,455,202 discloses a process for producing lithium by the electroreduction of a lithium compound dispersed in a fused salt electrolyte and deposition of the electroreduced lithium in a liquid metal cathode from which lithium is recovered. A fused salt electrolyte includes a lithium compound and at least one of the following elements of the Periodic Table including Group IIIA, such as boron; Group IVA, such as carbon; Group VA, such as nitrogen; and Group VIA, such as oxygen. The patent discloses the electroreducing of lithium oxide dispersed in such an electrolyte of fused lithium salts and alloying the electroreduced lithium with a liquid metal cathode. Suitable electrolytes are disclosed to be chalcogenides such as Li.sub.2 O, K.sub.2 O, CaS, Na.sub.2 S, K.sub.2 Se, CaSe, or BaTe; fluorides such as LiF, NaF, KF, CaF.sub.2, BaF.sub.2, or SrF.sub.2 ; hydroxides such as LiOH, NaOH, KOH, or Ca(OH).sub.2 ; sulfates such as Li.sub.2 SO.sub.4, Na.sub.2 SO.sub.4, K.sub.2 SO.sub.4, or SrSO.sub.4 ; nitrates such as LiNO.sub.3, NaNO.sub.3, KNO.sub.3, or Ca(NO.sub.3 ).sub.2 ; carbonates such as Li.sub.2 CO.sub.3, Na.sub.2 CO.sub.3, K.sub.2 CO.sub.3, CaCO.sub.3, or SrCO.sub.3 ; and mixtures of all the preceding.
The Hoopes cell process includes feeding an impure aluminum or aluminum alloy in a molten state as an anode in contact with a superimposed bath or electrolyte preferably containing one or more fused fluorides with or without the addition of chlorides, the pure aluminum being deposited on the cathode of molten aluminum preferably floating on the bath or electrolyte. The Hoopes cell is disclosed in U.S. Pat. Nos. 1,534,317 and 1,534,318. The preferred electrolyte or bath in the Hoopes cell contains 25-30% aluminum fluoride, 30-38% barium fluoride, 25-30% sodium fluoride, 0.5-3% alumina, and 2% calcium and magnesium fluorides present as impurities.
Aluminum-lithium alloys currently are receiving more attention as candidates for use in structural metal applications in the aerospace industry. Aluminum-lithium alloys offer the advantage of lighter weight and high structural integrity, making these alloys attractive to the aerospace industry for saving fuel.
Large quantities of scrap are generated for every pound of metal used in an aircraft. It is desirable to recycle most of this scrap into ingot form for further processing and application in new products, such as aircraft plate or sheet or aircraft extrusions. However, since several different alloys are used, mixed scrap may not be recyclable in whole or even in part by melting the scrap and forming the ingot directly. If scrap cannot be recycled into new aluminum-lithium ingot, some method must be found to remove and recover the lithium from the scrap. Processes that lead to the production of both lithium and lithium-free aluminum are desirable.
The electrolytic processes described above have one or more drawbacks or disadvantages which have been found to be undesirable in the pursuit of reclaiming lithium from aluminum-lithium alloy scrap.
It is an object of the present invention to provide a process for recovering the lithium content from recycled aluminum-lithium alloy scrap.
It is a further object of the present invention to provide a process for recovering lithium in a three-layered lithium transport cell.
It is yet another object of the present invention to provide a process for the recovery of lithium from aluminum-lithium alloy scrap in a lithium transport cell operating at predetermined process parameters of bath composition and temperature for improved process efficiency.
These and other objects of the present invention will become apparent from the detailed description of the invention as follows.